The invention relates to a friction clutch for a motor vehicle and especially to a friction clutch having a diaphragm spring.
Conventional friction clutches for motor vehicles, such as are known for example from DE-A-3,304,670, have a clutch cover secured on a fly-wheel of the internal combustion engine of the motor vehicle, a presser plate arranged axially between the fly-wheel and the clutch cover and guided non-rotatably but axially movably in relation to the clutch cover, and a clutch disc arranged between the presser plate and the fly-wheel and provided with friction linings. A diaphragm spring braces the clutch disc between the fly-wheel and the presser plate. The diaphragm spring has an annular basic body, which at least in the initially unstressed condition is of taper form, and a plurality of spring tongues protruding from the basic body radially towards the axis of rotation. In the clutch as known from DE-A-3,304,670 the diaphragm spring is loosely supported on the presser plate in the region of the external circumference of the basic body, while it is fixed on a circle of smaller diameter with the aid of securing means, both axially and radially on the clutch cover. A clutch releaser rests on the radially inner ends of the spring tongues. The releaser shifts the tongue ends of the diaphragm spring, installed with initial stress, against the stress force towards the fly-wheel, for the disengagement of the clutch. In the known clutch individual spring tongues are provided with outwardly domed portions by which ventilation openings are produced between adjacent spring tongues for the interior of the clutch.
Diaphragm springs ordinarily have a spring force/spring travel characteristic which firstly rises, starting from the relaxed condition, to a maximum, then falls off with increasing spring travel to a minimum, in order then to rise again. In order to achieve a minimum possible variation of the spring force during the life of the clutch, the diaphragm spring is usually installed with initial stress in such a manner that in the engaged condition it is situated on the far side of the maximum on the falling branch of the characteristic. This has the consequence that the spring force decreases with increasing spring travel towards the disengagement position. In a new clutch the spring force achievable in the disengaged position is at a minimum. With increasing wear of the friction linings of the clutch disc the clutch engagement position shifts more and more towards the maximum of the characteristic, with the consequence that the spring force in the clutch engagement position increases with growing wear.
It has appeared that the high rotation rates achievable with modern internal combustion engines exert centrifugal forces upon the spring tongues of the diaphragm spring which undesiredly influence the spring force characteristic of the diaphragm spring. As a consequence of their basic for example cranked form or by reason of the actuation by the clutch releaser, diaphragm spring tongues can be set obliquely of the axis of rotation, with the consequence that they are drawn outwards by centrifugal forces, and increase or decrease the spring force of the diaphragm spring according to the direction of their oblique positioning. In a new clutch, in which, as mentioned above, the spring force is at the minimum in relation to the life of the clutch, this can have the consequence that the return force of the diaphragm spring no longer suffices to engage the clutch again automatically. The clutch thus remains hanging. On the other hand towards the end of the life of the clutch, by reason of such an oblique placing of the spring tongues, in the engaged position the pressure application force can become unacceptably high.
It is already known from DE-A-1,450,163, in a friction clutch with a dished spring clamped in between the clutch cover and a presser plate, to lift away the presser plate by means of several clutch levers against the force of the dished spring. The clutch levers are mounted on the clutch cover through joints on the side of the clutch cover axially remote from the dished spring and connected by way of intermediate levers through the clutch cover with the presser plate. The clutch levers are designed so that their centre of gravity is offset in relation to the articulation axis in the axial direction of the clutch, so that the centrifugal forces acting upon the clutch levers act in the engagement direction and thus support the force of the dished spring in every pivotal position of the clutch levers.